Chlamydial polyarthritis-enteritis results from a complex host-parasite interaction in which an initial phase of an acute intestinal infection may lead to polyarthritis with chronic infection of synovial tissues and other sites of the body. The general objectives of the proposed investigations are: 1. To clarify on the cellular and subcellular level mechanisms of parasitism, leading to enteritis and polyarthritis. 2. To define the effect of chlamydial infection on the function of highly specialized cells such as enterocytes and synovial cells, and to assess the relation of infection to cellular function, differentiation and cell cycle as a basis for expression of disease. 3. To study arthropathic properties of chlamydial polyarthritis agents that determine their propensity for synovial tissues and their cytocidal effect. Using electronmicroscopic, virologic, immunologic, enzymatic, cytologic, and histochemical techniques, the effect of chlamydial infection on cellular organelles will be studied. Animals will be inoculated to study sequential changes in the development of enteritis and arthritis, and other phases will be explored in organ and tissue cultures. Infectious agents replicating during different phases of the mammalian cell cycle will be used.